INTERNAL PARASITES OF THE SEBAGO SALMON. II 9I 



identical in the two forms are such as are clearly fresh- water species, and that 

 the Sebago salmon contain at least one clearly fresh-water genus which is not 

 reported from the corresponding European host. 



Among the nematodes the evidence is less conclusive, since the amount of 

 material is smaller; indeed, hardly enough to form a basis for any conclusions. 

 At the same time, all the species which give to the parasitic fauna of the European 

 salmon its marine aspect are entirely wanting here. Not a single specimen of 

 Agamonema was discovered, although two species are found in the European 

 salmon, and one of them, Agamonema capsularia, is very common. Both Ascaris 

 and Echinorhynchus are unrepresented in the parasitic fauna of the Sebago 

 salmon. Among the numerous species of each already recorded as parasitic in 

 the European salmon three out of four are purely marine. Here again one 

 notes that the marine elements in the parasitic fauna of the European salmon 

 are wanting in the Sebago salmon. Possibly the large filariad found abun- 

 dantly in the Alaska salmon, and reported also from one or two salmon taken in 

 Sebago Lake, forms an exception to the general rule. As I have already noted, 

 it appears to be marine in origin. This may be, however, a false argument, 

 and the species may actually be one limited to this host or to the salmonid 

 family, regardless of habitat. In this connection one naturally recalls at once 

 the case of Abothrium crassum, which, from the observations on salmon in the 

 North Sea and then in the Rhine, might be said to be a marine form, since it 

 gradually disappears on the journey up the Rhine. But it occurs in hosts of 

 purely fresh- water habitat, such as Salmo hucho in Europe and Cristivomer 

 namaycush in the Great Lakes of North America. Evidently further informa- 

 tion is needed before one can safely assign this nematode to a definite habitat. 



Summing up all the evidence concerning the parasites of the Sebago salmon, 

 one finds that four species are unknown in character, one only is possibly marine, 

 one is a pure salmon parasite, and three are clearly fresh-water forms. The 

 latter are also its most frequent and numerous guests. Furthermore, the 

 Sebago salmon lacks every one of those parasites found in the European salmon 

 which must be regarded as purely or largely marine, and possesses in common 

 with its European congener only one characteristic salmon parasite and possibly 

 also two fresh-water forms, which, though abundant in its own parasitic fauna, 

 are very rare in that of its relative. 



The parasitic fauna of the Sebago salmon manifests a striking fresh-water 

 aspect, all the more unexpected in view of the marine character of that in the 

 European salmon as demonstrated by Zschokke. One could hardly find a more 

 convincing demonstration of the fundamental biological relation between parasite 

 and host. 



The parasitic fauna of any animal is primarily a function of its habitat. 



